This invention relates to variable focal length lenses. There is a need for such lenses as spectacle lenses because, as a result of aging, the lens in the human eye becomes incapable of sufficient accommodation to focus on near or intermediate objects. After the onset of the condition of limited focal accommodation, called presbyopia, a single set of fixed focus spectacles will be found to be unsatisfactory for both distant and near vision, irrespective of the wearer's general visual acuity. Whatever correction (if any) may be required to correct a person's vision for distance, an additional amount of optical power (up to about three diopters) will be found to be required to correct that person's eyesight for near vision. The required "near addition" generally does not involve an astigmatic component.
The usual solution to this problem is to fit persons suffering from presbyopia with spectacles having bifocal lenses. In the most common form of bifocal lens, the upper part is ground to provide the wearer with the proper correction (if any is needed) for distance vision, and the lower part is ground with the same correction plus a relatively small near addition. The near addition generally does not exceed a few diopters of optical power. Using bifocal lenses in a pair of spectacles allows a person to see distant objects clearly by looking straight ahead, and to see close objects clearly by looking downward.
The bifocal solution is not entirely satisfactory for at least three reasons. Firstly, many people have difficulty in adapting to bifocals; secondly, there is often the need to see near objects which are on a level with, or above, the wearer; and finally, there is usually a mid distance range within which which neither part of the lens is satisfactory. Trifocals or progressive multifocal lenses are used to help alleviate this last problem, but the first two difficulties remain, and in addition, the in-focus field of view may be more limited than is desired. The foregoing problem was addressed in a prior patent issued to one of the present inventors, U.S. Pat. No. 5,138,494. This patent discloses a continuously variable focus lens which can be adjusted by the wearer to focus on any object, irrespective of its distance from the wearer.
In the aforementioned prior patent, a liquid-filled variable focus lens was disclosed which includes a distensible transparent membrane spaced from a rigid lens, with the space between them filled with a liquid having a relatively high refractive index. The membrane is cemented to a membrane support (which is in the form of an annular rim), and the peripheries of the rigid lens and the membrane support are connected with a flexible sealing member. The rigid lens, the membrane, and the sealing member define a substantially fixed volume for the liquid filling. Changing the spacing between the membrane support and the rigid lens in such a structure causes the membrane to assume a curved form, either increasing the power of the lens or decreasing it, depending on the direction of the change in spacing. If the periphery of the membrane is circular, its shape, when distended, will be spherical, and no optical distortion will be encountered in use. However, one very substantial application of the lenses described in the previous patent is in variable focus spectacles where, for reasons of style, shapes other than circular are often desired. It has been found that in such cases, depending on the extent of the non-circularity of the lenses, the membrane shape may deviate significantly from the desired spherical shape, and greater than desired optical distortions may be encountered.
It is therefore an object of the present invention to enable the production of a liquid filled variable focal length lens wherein the optical distortion arising from non-circularity of the lens may be minimized.